1. Field of the Invention
The present invention relates to devices for use in the rotational alignment of non symmetrical articles, such as non-cylindrically symmetrical optical elements which transmit, emit or receive polarized light. More particularly, the present invention relates to a connector with an internal structure which may be used to optimize the rotational alignment of interconnected optical elements and maintain the polarization of light transmitted between them. Even more particularly, the present invention relates to a connector for non-cylindrically symmetrical optical fibers which includes multiple internal alignment features asymmetrically spaced about the connector axis. When the connector of the invention is utilized to connect a first non-cylindrically symmetrical optical fiber to a second non-cylindrically symmetrical fiber, the alignment features negate or minimize small alignment errors and allow the assembler to optimize the rotational alignment of the interconnected optical fibers with respect to a reference feature on the connector housing. The fibers may then be interconnected while maintaining accurate alignment of their polarization axes.
2. Description of Related Art
Optical elements, such as optical fibers, laser diodes and other light sources, polarizers, lenses, beam splitters and the like, are presently in wide use, particularly for high speed communication and data transmission. Connectors may be used to non-permanently connect, disconnect and reconnect the optical elements incorporated into an optical communication network, while splices may be used to permanently connect the network elements. The present application is directed to connectors, which may be easily coupled and uncoupled to allow multiple, non-permanent connection and reconnection of optical elements.
Many such connector designs are in present use. As explained in detail in U.S. Pat. No. 5,321,784, standardized connector designs used in telecommunications applications include the ST connector, the SC connector, and the FC connector. Regardless of the design selected for a particular application, alignment of the terminal ends of the connected optical elements is critical to maintain the signal strength as the light passes through the connection. To connect standard telecommunications grade optical elements, such as optical fibers (which essentially have cylindrical symmetry), the fibers must be supported and oriented both longitudinally and transversely to minimize attenuation of the light signal passing through the fiber connection. As is well known in the art, this is accomplished by optimizing fiber positioning to ensure minimum transverse and longitudinal offset between the fiber cores.
In contrast to the optical fibers utilized in the telecommunications industry, some optical fibers are non-cylindrically symmetrical. They may have non-circular cross sections, or may simply have performance that depends on their rotational orientation. Some non-cylindrically symmetrical fibers are constructed so that they are birefringent, that is, light of different polarizations will have different propagation characteristics within the fiber. For example, polarization maintaining (PM) and polarizing (PZ) fibers have a non-cylindrically symmetrical internal structure designed to maintain the polarization of the light in the fiber. Such fibers typically have two transverse axes associated with this polarization birefringence.
If plane polarized light is launched into the fiber so its plane of polarization coincides with a transverse axis of the fiber, the polarization of the light is maintained as the light propagates down the length of the fiber. When making a connection between two non-cylindrically symmetrical optical fibers, it is important to rotationally align the transverse axes of the two fibers accurately so that the polarization state will be preserved.
A measure of the performance in a polarization maintaining fiber optic system is polarization crosstalk, sometimes referred to as extinction ratio, which is defined as the ratio of the optical power of the light in the undesirable polarization state to the power of the light in the preferred polarization state. This ratio is related to the rotational orientation offset: ##EQU1## where .beta. is the angle between the axes of the two fibers being connected, P.sub.x is the optical power in the preferred polarization state, and P.sub.y is the optical power in the orthogonal polarization state. The value of the crosstalk is commonly expressed in dB: ##EQU2##
It is desirable to have a polarization crosstalk of less than -30 dB. This level of performance requires alignment accuracy within about 2.degree.. Unfortunately, there are also many other factors that adversely affect the polarization crosstalk. Therefore, in practice, it is desirable to have orientation accuracy of better than about 1.degree.. Such high precision is very difficult to achieve with conventional connector designs.
Two principal existing optical fiber connector designs which have been used primarily for standard cylindrically symmetrical telecommunications fibers, FC and SC, are also generally suitable for use as connectors for non-cylindrically symmetrical fiber elements, such as PZ or PM optical fibers. Both these connector types normally include a projection or key on the connector which mates with a detail on the associated adapter, thus fixing the rotational alignment of the connector relative to the adapter and thus to a second connector. It is this control of rotational alignment of the connectors which makes SC and FC connectors suitable candidates for connectors of polarization-controlling optical fibers.
Some known SC and FC connector designs have limited rotational orientation capability. However, these designs are intended merely to reduce excess loss by the accommodation of eccentricity of fiber cores with respect to the connector bodies in the connection. For example, in U.S. Pat. No. 5,016,970 Nagase et al. discuss the problem of optical fiber misalignment and provide symmetrically opposed keyways in a ferrule body as a means of alignment. These keyways interlock with symmetrically opposed keys formed in a plug housing which receives the ferrule. The keys and corresponding keyways allow two possible orientations of the optical fiber in the plug housing. The alternate positions are symmetrically arranged and have a 180.degree. separation. Stephenson et al. in U.S. Pat. No. 5,212,752 disclose a connector that has enhanced provisions for tuning eccentricities of an optical fiber core or fiber receiving passageway in a ferrule. Stephenson et al. minimize the influence of eccentricity by providing an SC-type fiber connector that may be united to a second fiber in a variety of fixed alignments. A series of trial connections, made using alternate fixed fiber alignments, eventually yields the lowest loss connection for a given connector. The limited orientation capability in the Nagase and Stephenson connector designs does not provide the accurate rotational orientation adjustment which is required when connecting non-cylindrically symmetrical optical fibers. In addition, conventional connector designs fail to include means to minimize or compensate for small alignment errors and manufacturing inaccuracies that would otherwise result in rotationally misaligned fibers.
Rotational alignment of polarization maintaining optical fibers is discussed in some detail by Nagase et al. in U.S. Pat. No. 5,216,733. The '733 patent discloses a method for aligning a fiber-beating ferrule with a keyway formed in a ring-shaped flange and thereafter bonding the ferrule in position relative to this flange. Fiber alignment is made while observing the polarization orientation of light passing through the polished end of the optical fiber. Installation of the ferrule in the plug housing is limited to one of two rotationally equivalent positions determined when the internal key on the housing mates with the keyway of the ferrule flange.
The rotational symmetry of the PM optical fiber in the '733 patent matches the rotational symmetry of the associated connector housing. Therefore, the diametrically opposed keyways in the alignment flange, which are symmetrically spaced about the principal axis of the housing, provide rotationally equivalent positions for the retained fiber and contribute no added alignment benefit when the flange is engaged with the housing. The connector design described in the Nagase '733 patent thus fails to include means to minimize or compensate for small alignment errors and manufacturing inaccuracies.